In general, an ignition coil for an internal combustion engine such as a vehicular engine includes a coil portion having a coil case accommodating a primary coil and a secondary coil. The coil portion has one axial end provided with a plug mount portion, which is equipped with an insulator portion of a sparkplug. The coil portion and the plug mount portion are connected and located on the same axis. Such an ignition coil is mounted to a cylinder head cover of an engine. Here, various constraints are subjected to a mount space, in which the ignition coil is mounted to the cylinder head cover. In a structure where such a plug mount portion of an ignition coil is equipped to a sparkplug attached to the cylinder head, a coil portion of the ignition coil needs to be located in the limited space in a cylinder head cover. In this case, when the ignition coil includes the coil portion and the plug mount portion located on the same axis, the outer circumferential periphery of the coil portion and the cylinder head cover therebetween define a reduced clearance. Consequently, the outer circumferential periphery of the coil portion may interfere with the cylinder head cover due to vibration of the engine.
For example, U.S. Pat. No. 5,026,294 (JP-U-2-115969) discloses an ignition coil, in which an axis of a plug socket (plug mount portion) is offset from an axis of a coil portion. Alternatively, for example, JP-A-10-220331 discloses an ignition coil having an axis portion provided with a socket portion, which is to be mounted to an igniter plug. The socket portion is formed of elastic resin and bendable with respect to the axis portion. In the present structure, the igniter plug is inclined relative to an axis of the cylinder of the engine. A socket portion of the ignition coil is configured to bend along the axis of the igniter plug when the socket portion is mounted to the igniter plug along a direction in parallel with the axis of the cylinder. In the present structure, a bracket of the ignition coil can be provided perpendicularly to the axis of the cylinder, whereby a shape of the bracket can be restricted from being complicated. However, both U.S. Pat. No. 5,026,294 and JP-A-10-220331 do not show a structure configured to appropriately secure a clearance between the coil portion and the cylinder head cover.
An ignition coil for an engine includes a primary coil and a secondary coil. The primary coil energizes in response to an instruction from an electronic control unit (ECU). The secondary coil generates high voltage (secondary voltage) for generating a spark in response to a change in magnetic flux produced when the energization in the primary coil is terminated. In a structure of a dual ignition system (two-point ignition system) in which mixture gas is ignited at two locations in a combustion chamber of each cylinder, two plugholes are provided to a cylinder head for each cylinder. In this dual ignition system, an ignition coil is mounted to each of the two the plugholes. For example, according to an igniter for an internal combustion engine disclosed in U.S. Pat. No. 6,189,522 B1 (JP-A-11-230017), sparkplugs are electrically connected respectively with one winding end of a secondary coil and an other winding end of the secondary coil. In the present structure, when energization of a primary coil is terminated, magnetic flux changes to simultaneously generate sparks between electrodes in each of two sparkplugs. In an ignition device of JP-A-2001-12337, for example, two secondary windings (secondary coils) are integrated with one primary winding (primary coil) to apply voltage, which is opposite from each other in polarity, to two electrodes. However, a space mounted with an ignition coil is limited in each cylinder. Therefore, in a two-point ignition system in which two ignition coils are mounted to each cylinder of the engine, a distance between two plugholes in a cylinder head needs to be increased. In particular, a distance between plugholes is significantly small in an engine with a small boa diameter of a piston. Consequently, two ignition coils are hard to be mounted to each cylinder.
An ignition coil for an engine includes a primary coil and a secondary coil. The primary coil energizes in response to an instruction from an electronic control unit (ECU). The secondary coil generates high voltage for generating a spark in response to induced electromotive force produced when the energization in the primary coil is terminated. A center core formed of a soft magnetic material is provided on the radially inner side of both the primary coil and the secondary coil. An outer core formed of a soft magnetic material is provided on the radially outer side of both the primary coil and the secondary coil. The cores construct a magnetic circuit for conducting magnetic flux generated by the primary coil. For example, according to an ignition coil of JP-A-2002-31025, an outer core and a center core configure a magnetic circuit for conducting magnetic flux, which is generated by a primary coil. The outer core defines a closed magnetic path. The center core passes though the inside of the outer ring core. One end of a center core and an inner circumferential periphery of the outer ring core, which is opposed to the one end of the center core, therebetween define a core gap. The core gap is provided with a piece of a permanent magnet. In the present structure, change in magnetic flux in each iron core can be enhanced at the time of termination of energizing of the primary coil, whereby an output of the ignition coil can be enhanced. In the ignition coil of JP-A-2004-169619, for example, multiple silicon steel plates are stacked to construct an E-shaped iron core and an I-shaped iron core. The E-shaped iron core and the I-shaped iron core are used to construct a magnetic circuit defining a closed magnetic path. However, in each of JP-A-2002-31025 and JP-A-2004-169619, a suitable structure of the closed magnetic path core being in the square annulus shape is not disclosed.